Biological Applications
Typically, biological electrical signals, such as electroencephalographic (EEG), electromyographic (EMG), and electrocardiographic (ECG) signals, are sensed with contact electrodes.
Contact electrodes typically require a moist, electrically conductive, paste be present between the electrode and skin surface in order to maintain low contact resistance. This paste, and associated contact electrodes, can cause difficulties ranging from allergic contact dermatitis and skin staining to intermittent connections as the paste dries. Contact electrodes may require skin be abraded during attachment, causing discomfort and irritation, and callus formation if frequently repeated. Open circuits often result as electrodes shift due to patient movement. Electrodes must also be either disposable, or carefully cleaned between patients to avoid transmission of infection.
Many electrodes are typically required for such applications as EEG and polysomnography, requiring much time and care to attach to patients in correct patterns. Even the classical “12-lead” ECG commonly used in medical practice requires that a dozen electrodes be attached to the patient.
The problems of contact electrodes are aggravated by long-term use. Polysomnography requires patients be wired for EEG, EMG, and ECG signals for a full night's sleep of at least 6 hours; Holter monitors typically require at least 24 hours of ECG monitoring; and patients may desire to wear EMG-controlled prosthetics as much as 18 hours a day for months or years. It may be useful to monitor epileptics for seizure activity or sub-seizure EEG disturbances over periods of weeks while medication doses are adjusted. Such long term use gives ample opportunity for contact dermatitis with contact electrodes, especially in those people who are already sensitized to metals such as nickel or silver.
Experiments have shown the possibility of using EEG signals to control assistive devices for the severely handicapped, and suggestions have been made that there may be a market for EEG-controlled toys or computer games. Long term use of contact electrodes with conductive gels is undesirable in these applications.
It is therefore desirable to have noncontact sensors for microvolt and millivolt-level electrical field signals that do not require use of the traditional conductive gel or paste between sensor and skin.
A Mach-Zehnder interferometer fabricated on a lithium niobate substrate has been used for non contact or minimal-contact, high-impedance, electric field sensing, as described in U.S. Pat. No. 6,871,084. A conductive rubber pad with a saw tooth surface was used with this sensor to contact the scalp through the hair for EEG monitoring, while the larger ECG signals were measured through light clothing with no direct skin contact. This is, however, a fairly large and complex device, and was of marginal sensitivity for EEG signals.
Surface Plasmon Sensing
The Surface Plasmon effect involves electromagnetic radiation propagating as quantized collective oscillations of the “free electron gas” of a metal along a metal-dielectric interface, where these free electron gas oscillations can couple to photons as “polaritons”. Such coupled radiation can be guided by structures as small as 10% of the free-space wavelength of the electromagnetic radiation.
The propagation of surface plasmons is sensitive to a variety of surface-related effects, including changes to the local index of refraction of adjacent materials. This effect has been used in sensors that measure characteristics of proteins bound to a metal surface.
Surface plasmons at a metal-dielectric boundary can be excited in the Kretchmann-Raether configuration by directing a beam of light through the dielectric onto the boundary at an angle such that most incident light is reflected. Surface plasmons can also be created by guiding electromagnetic radiation through a tapered waveguide of dielectric adjacent to the metal.
Electromagnetic radiation also couples to surface plasmons where a grating on the metal surface has dimensions on the order of a wavelength of the radiation, as described in U.S. Pat. No. 4,765,705. Similar structures can also couple energy from plasmons back into electromagnetic radiation. Gratings on the metal surface of appropriate dimensions, such as a grating having spacing equal to a multiple of half a wavelength, can also reflect plasmons.
It is known that certain electro-optic materials, such as lithium niobate crystals, have an index of refraction that varies with an applied electric field.